Synchronizing clocks across a communication link

ABSTRACT

A slave clock may be synchronized to a master clock by means of a synchronization signal sent from the master to the slave clock side of the link. The synchronization signal may be an expected signal pattern sent at intervals expected by the slave side. The slave clock may correlate received signals with a representation of the expected synchronization signal to produce a correlation sample sequence at a first sample rate which is related as n times the slave clock rate. A best interpolation may in turn be further refined by estimating between interpolator outputs adjacent to the best interpolation output. The synchronization signal receipt time thus determined is compared to the expected time based upon the slave clock, which is adjusted until the times match. The best interpolation may in turn be further refined by estimating between interpolator outputs adjacent to the best interpolation output.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/170,391, filed Jun. 29, 2005, which is a divisional of U.S. patentapplication Ser. No. 09/790,443, filed Feb. 21, 2001 and issued Sep. 13,2005 as U.S. Pat. No. 6,944,188, which are hereby incorporated in theirentirety by reference.

FIELD OF THE INVENTION

This invention relates to data communication systems, and tosynchronizing independent clocks between nodes of such systems.

BACKGROUND

Data communication systems typically transfer data from a source to anend user by routing the data in packets through a series of nodesconnected by links. It is generally faster to transfer datasynchronously, when circumstances permit, and yet in many instancescommunication system links do not share an explicit clock. In thetypical circumstance that the clocks at different node are independent,and yet synchronous data transfer timing is desired. Moreover,demodulating a modulated data signal can be done more reliably if thetiming of the signal is precisely known by the receiving device.Although it is possible to independently synchronize the timing of eachtransmission block, timing certainty can be enhanced by synchronizingthe clocks on each side of a link, even across different transmissionblocks. Thus, there is a need for an apparatus and method to synchronizeclocks across a communication link.

In some circumstances, such as when links operate in half-duplexcommunication modes, information is not provided continuously from atransmitter having a master clock to a receiver having a slave clock,and indeed such periods of non-transmission may be variable. Therefore,it will be advantageous for a clock synchronization mechanism used insuch circumstances to establish and retain a lock despite an absence ofinformation for substantial periods of time compared to the clockfrequency.

Some forms of data transmission occur at a particular rate. For example,a DS1 (or T1) voice connection provides 193 bits every 125 microseconds,as determined by a “network clock” used by a source of such data. It isoften important for an entity receiving such data to process it at arate which precisely matches the sending rate. One method to match theprocessing rate is to process the received data under control of a clockwhich matches the network clock. This presents a further need tosynchronize clocks which are otherwise independent.

A transfer clock (symbol clock) and a network clock are typicallyseparate clocks which are independent of each other. A particularcommunication link may have both types of clocks. If the two clocks insuch a link are independently maintained at each end of the link, then aneed arises for synchronization of both clocks in the same link.

In the situation where a common clock (such as a GPS clock) is availableto both sides of a network connection, it is known to use such commonclock to synchronize a slave clock to an independent master clock, aspresented in “Synchronous Techniques for Timing Recovery in BISDN” byLau, et al., IEEE Transactions on Communications, Vol. 43, No. 2/3/4,February/March/April 1995. This approach is useful only when a commonclock is available. Moreover, the technique as presented cannot reliablybe used to phase-lock clocks in the presence of unknown transmissionphase delays.

Accordingly, there is a need to synchronize clocks across acommunication link when no common clock is available, and a need fortightly synchronizing clocks across a communication link to enhance thespeed and accuracy of data transfers across that link.

SUMMARY OF THE INVENTION

The above needs are addressed herein by providing a system, methods, andapparatus to synchronize one or more pairs of initially independentclocks over a communications link. Separate clock pairs may besynchronized by different techniques. For example, one can firstphase-lock a “transfer” clock pair (e.g. a modem symbol clock), and thenrely upon the locked transfer clock to subsequently synchronize anetwork clock pair.

This is useful in any communication link which needs to synchronizeclocks. A communication link in the form of a broadband wireless linkconnecting a plurality of end users to various networks is described asan example. The broadband wireless link needs to demodulate anintermittent signal containing data. To do so, it must synchronizesymbol detection to the modulated symbol transmission. Suchsynchronization is simplified if a clock indicative of symbol timing islocked, so that the detector always knows symbol timing, even prior tothe beginning of a transmission block. Moreover, the more tightly thesymbol timing clocks are locked, the faster and/or more accurate thedetection can be.

The wireless link may receive data in Asynchronous Transfer Mode (ATM),which as its name implies is an inherently asynchronous communicationprotocol. However, the ATM data may convey data which is being providedfrom a source (such as a DS1 connection) at a constant bit-rate (CBR).After transfer across the link, the data will be further forwarded, alsoat a constant bit-rate. If the rate of the source of CBR data does notmatch the rate of the forwarding of the CBR data, then system bufferstemporarily storing the data will either overflow or underflow. Thus,the output and input data rates should be synchronized to prevent dataerrors. These data transfer rates are controlled by “network” clocks.One way to ensure that the output rate is the same as the input rate isto synchronize the pair of clocks, one at each end of the link, whichreflect or control network timing at their end of the link. However, thenetwork clock on one side of the link does not have direct access to thenetwork clock on the other side of the link. The problem created isessentially a need to synchronously convey information over aninherently asynchronous communication link. Thus, the wirelesscommunication link system may advantageously use both transfer clocksynchronization and network clock synchronization.

The invention can be practiced consistently with the general frameworkof the Media Access Control (MAC) protocol, as defined for example in“Media Access Control Protocol Based on DOCSIS 1.1,” submitted Dec. 22,1999 in connection with IEEE 802.16 Broadband Wireless Access WorkingGroup and incorporated herein by reference, and is expected to beuseable within the framework of the IEEE 802.16.1 MAC when that standardis defined. Some embodiments diverge from aspects of MAC protocols aspresently known or proposed. In addition to embodiments within a MACprotocol framework, however, those skilled in the art will understandthat the present invention may be practiced in any communication systemhaving independent clocks which need to be synchronized to facilitatesynchronous data transfers and/or consistent data transfer rates acrossa link.

Particular embodiments of the present invention include a millimeterwave wireless RF channel communications system which connects singlebase stations each to a plurality of relatively proximate CustomerPremise Equipment (CPE) stations. A network of such base stations withtheir surrounding CPEs can provide all communications services over alarge area, such as a city. This system is representative of a varietyof present and future communication systems which have links joiningnodes which do not share an actual clock. For such systems, thepresently existing synchronization techniques are not optimal, and theimprovements in synchronization taught herein enable more accurateand/or faster data transfer.

Embodiments of the present invention include methods, systems andapparatus for synchronizing a slave first clock to a master first clock.Information from the master about a timing relationship between themaster first clock and a master second clock may be used to synchronizea slave second clock to the master second clock. The first clocks may bemodem symbol clocks, or transfer clocks; the second clocks may benetwork clocks which reflect data transfer rates. The first clocksynchronization may include phase-locking, even to within one thirtysecond of a symbol clock period, and the second clock synchronizationmay be merely frequency matched. Transmission between the master and theslave may be discontinuous, with periods of variable length betweentransmissions. Phase locking the first clocks may require transmissionof an expected preamble at an expected time according to the masterclocks, and adjustment of clock operation at the slave until theexpected preamble arrives at precisely the expected time according tothe slave clock. The expected preamble may be compared to the receivedpreamble by a correlation method or circuit, which may employ one ormore correlations and one or more interpolations of the correlation.

The most detailed example herein involves communication network nodesseparated by a millimeter-wave radio link over which data iscommunicated bidirectionally using time division duplexing (TDD). SinceTDD utilizes the same frequency for both uplink and downlinkcommunications, the transmissions in each direction are receiveddiscontinuously. That is, each receive period is interrupted by atransmit period. Clock synchronization is made more difficult in thiscircumstance because receipt of clock timing indications disappearduring these transmit interruptions, which are of a variable, thoughbounded, duration. The system most detailed herein also synchronizes twoseparate clocks—a modem symbol clock, and a network clock—across thelink. However, it should be kept in mind that the present invention maybe embodied in any communication system which has links joining nodeswhich do not share an actual clock, but which desire to synchronize oneor more clocks in order to enhance data transfers.

A master side establishes a master transfer clock or a master networkclock, and transmits information reflecting one of those clocks to aslave side. In the case of a master transfer clock, the informationpreferably includes a predetermined data stream which is sent a knownquantity of transfer clock periods after a preceding data stream wassent. In the case of a master network clock, the information preferablyincludes numeric data reflecting a timing relationship between themaster network clock and the transfer clock local to the master networkclock side of the communication link.

A slave side receives information, presumably from a master side,according to which it adjusts a slave transfer clock or a slave networkclock. In the case of a slave transfer clock, the received informationpreferably includes periodic bursts of an expected data patterndelivered at intervals separated by some number of periods of the slavetransfer clock. The slave side determines the exact arrival time of theexpected pattern, and from this information modifies the slave transferclock so that its frequency tracks the timing indicated by the receiveddata pattern. The slave transfer clock is adjusted until the number ofperiods of the slave transfer clock between pattern arrival times is asexpected. The slave transfer clock may phase lock upon the mastertransfer clock which is reflected in the timing of the received datapattern. In the case of a slave network clock, the received informationincludes data, and the slave network clock frequency is adjusted untilit has a relationship to its local transfer clock which comports with arelationship indicated in the received data.

A plurality of clock pairs may be synchronized according to the teachingof the present invention, and thus “first” clocks and “second” clocksare often referred to. However, it is sometimes instructive to refer toa concrete example rather than the most general case. Therefore,references to “symbol” clocks, “transfer” clocks, and “primary” clockswill be used somewhat interchangeably with “first” clocks. Similarly,“network clocks” will be used somewhat interchangeably with “secondclocks.” It will be appreciated by those skilled in the art that anysynchronization technique may be used to synchronize any particularclock pair, and that the designation of the clock type (e.g. symbol,transfer, or network) is merely exemplary, and is not intended to belimiting, but rather to provide a more concrete description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a network carrying data, some at fixed rates, synchronouslyover links.

FIG. 2 is a system for synchronizing two independent clock pairs acrossa communication link.

FIG. 3 shows sectorized communication links between a Base Station andCPEs.

FIG. 4 shows indoor unit apparatus implementing Base Station functions.

FIG. 5 shows master and slave clock synchronization system modules.

FIG. 6 is a more detailed block diagram of the slave symbol clocksynchronization system.

FIG. 7 is a block diagram of the correlator of FIG. 6.

FIG. 8 is a block diagram of the digital loop filter of FIG. 6.

FIG. 9 is a block diagram of the peak detector of FIG. 6

FIG. 10 represents a noncommon clock error calculator circuit.

FIG. 11 is a flow chart for implementing slave network clock control.

FIG. 12 details a sigma-delta DAC for use with the slave network clockcontrol.

FIG. 13 shows a partitioning of network clock control tasks betweenhardware and software.

DETAILED DESCRIPTION 1. Data Links and Network Clocks

Data typically travels through a network in packets, from node to nodeacross links. A data source can be identified, though not necessarilythe original source, which determines the rate at which the data isreceived. Referring to FIG. 1, a Source_(A) 102 provides data 106 to thenetwork 130. Data 106 includes some fixed-rate data streams, such as aDS1 (also called T1) telephony connection. A network clock 104, CNet_(A)which is local to Source_(A), determines the precise rate at which suchdata is provided into the network 130. A Source_(B) 108 represents asource of data 110 which is not necessarily supplied at a fixed rate,and may include for example Internet Protocol (IP) packets. The datafrom these sources is merged at node 112. Node 112 may be a switch whichreformats the incoming data into a format such as ATM cells. In anyevent, the combined data 114 is then transferred to another node 116,from whence the data is distributed appropriately to users. From node116, data 118 is conveyed to first user 126, data 122 is conveyed touser 124, and data 126 is conveyed to user 128.

The DS1 data 106 is delivered to user 124 from node 116 as part of thedata 122, where it may be distributed to reconstruct a number ofseparate voice and/or one or more data connections. In order to preventoverflow or underflow of buffers at the user 124, it is important thatthe user 124 deliver the data 106 at a rate which precisely matches therate at which it arrives from Source_(A). However, the Network Clock 104at Source_(A), which determines that data rate, is not available to theuser 124, and it will be helpful to reconstruct a clock CNet_(D) 132which matches at least the frequency of CNet_(A). The nodes 112 and 116may also need to reconstruct the clock CNet_(A) 104. (It is alsopossible that the network clock CNet_(A) which controls the rate of therepresentative fixed-rate data 106 is in fact local to the node 112, oreven to the node 116.)

2. Modem Link Symbol Clocks

A data link connection between two nodes is likely to convey data bymodulating a high-frequency carrier in a modem at one end anddemodulating the modulated carrier at the other end. Such a modemconnection will be assumed between node 116 and the user 124. A modemconnection will generally have a clock to determine the “symbol” rate atwhich symbols are encoded or modulated on the carrier. Such a clock,denoted here CSym, will generally be independent of network clocks,CNet. It will substantially facilitate data transfer across a modemconnection, such as between node 116 and user 124, if both sidesprecisely know CSym. Thus, in addition to a Network Clock CNet whichwill advantageously be synchronized between nodes of a network, there islikely to be a separate Symbol Clock CSym which needs to be synchronizedbetween any two nodes which are communicating by means of a modulatedcarrier (i.e. through modems).

The ensuing description refers to master clocks on one side of acommunication link, and slave clocks on the other side, with each slavebeing adjusted to match the master. A plurality of such clockmaster-slave pairs may exist across a given link, and in one aspect theinterrelationship between at least two such pairs is described. Themaster and slave sides of communication links across which such clocksare being synchronized are separately identified in the figures in aneffort to reduce confusion between the plural types of master-slaverelationships. Exemplary links which may employ the apparatus, system ormethods taught herein are defined, for example, between nodes 116 and124 of FIG. 1, where the link conveys data 122; in FIG. 2, the master200 and slave 250 are at opposite side of a link 240 which conveysinformation 242; and in FIG. 3, base station 300 is one node and a CPE,e.g. 320, is the other node, with the link in that case being 342. InFIG. 3, a multiplicity of such links is shown, each having base station300 as one node. In FIG. 5, master 552 occurs at one node while slave550 occurs at the other node; the link is between modems 516 and 552.Thus in each case, the synchronization of clocks is occurring betweennodes across a communication link. Keeping this correspondence of nodesin mind, we turn to details of the specific figures.

FIG. 2 shows a combined master and slave system for synchronizing both aNetwork Clock pair and a Symbol Clock pair across a communication linkfor connecting nodes in a communication system. One node constitutes amaster system 200, which is the “master” as between network clocksCNet_(X). The master system 200 includes an incoming data module 202which is configured to accept data from a network connection (notshown). The incoming data generally includes at least some constant bitrate (CBR) data arriving at a particular rate, and the system mayinclude a first network clock module 216 configured to establish anetwork clock, CNet₁, reflecting that rate. The clock may beestablished, for example, by direct input from a source of the CBR data,or may be developed in master system 200 to match the actual rate atwhich CBR data is received. Module 216 is at least configured to makeCNet₁, available to the Master 200, even though CNet₁, may be dependenton a remote source.

Before transferring the data 202 to the Slave 250 across the Modem Link240, the master system 200 adds data from a control information module204, and data from other sources 206. Control information module 204 mayin particular provide data indicating a relationship between CSym₁, andCNet₁, to permit the slave CNet₂ to be adjusted to match CNet₁. Thisprocedure is described in more detail below. The data is combined into asingle bitstream in multiplexer module 208. The multiplexed data 210will be transferred across the connection, Modem Link 240, via amaster-side modem module 214, at a rate determined by a symbol clockCSym₁, which is developed in symbol clock module 212. The symbol clockin the symbol clock module 212 of the master system 200 need not be the“master” as between the symbol clock of the two nodes; but if it is,then it may merely provide a simple fixed clock at an appropriatefrequency, and its frequency will be used by the master-side modemmodule 214 to send a signal across the link reflecting the symbol clockfrequency. However, it may also be the “slave” of the symbol clock inslave system 250, in which event it would include the functionalitydescribed below with respect to the slave system 250. Thus it will beunderstood that “master” and “slave” symbol clock functions may beinterchanged across the communication link. The modem module 214 acceptsthe data 210, modulates it as a signal, and conveys it across the linkas signal 242 on a media 240, which may be RF spectrum in a wireless RFlink.

The modulated data signal 242 is received and demodulated in aslave-side modem module 252, under the control of a symbol clockprovided by slave-side symbol clock module 254. If the slave-side symbolclock is also the “slave” (which need not be the case), then it will beconfigured to be adjusted in view of the signal indicating the timing ofthe master-side symbol clock which will be received with modulatedsignal 242. In order for the signal 242 to convey data across the linkvia media 240 with the highest speed and lowest error rate, it isdesirable that CSym₂ 254 be precisely synchronized to CSym₁. It ispreferred that these two originally independent clocks be phase-lockedto within a small fraction of a symbol, preferably within not more than½ symbol, more preferably within ⅛ symbol, and even more preferably moretightly yet, for example within 1/16 or 1/32 symbol. The more consistentis the phase relationship between these clocks, the more reliable can bethe demodulation of signal 242. Details of the methods and apparatusused in the slave symbol clock module 254 to phase lock the symbolclocks across the link as stated above is shown below, particular withregard to FIGS. 4-9.

Demodulated data 256 is provided to controller module 258, which amongother tasks is configured to sort the combined data into control data260 and outgoing data 262. Control data 260 may be used, for example, tocontrol CSym₂ in slave-side symbol clock module 254 and/or CNet₂ inslave network clock module 264, and for other tasks which facilitate thetransfer of data across the Modem Link 240. In particular, the controldata 260 may include data reflecting a relationship between CNet1 andCSym1. The outgoing data 262 will be delivered from the Data Out buffer266 as Data Out 268 at a rate controlled by CNet₂ 264. It is desirablethat CNet₂ 264 at least match the frequency of CNet₁, 216 so as toprevent overflow or underflow of data buffers handling Data Out 266.Accordingly, in slave network clock module 264 the data reflecting therelationship between CNet₁, and CSym₁, may be compared to a relationshipdetermined between CNet₂ and CSym₂, and the frequency of CNet₂ adjustedso that the relationships are matched, thereby synchronizing CNet₂ toCNet₁, by leveraging the previous synchronization of CSym₁ to CSym₁.

It will be understood by those skilled in the art that communicationstake place in both directions across Modem Link 240. The designation ofMaster and Slave in FIG. 2 reflects that the clock CNet₂ on the slaveside is derived from the clock CNet₁, on the master side. The symbolclocks CSym₁, and CSym₂ also have a master and slave relationship toeach other, but either side may have the symbol clock. which is used asa master; this will typically be the base station side, which typicallycommunicates to a plurality of other nodes (only one such connection isshown in FIG. 2 for simplicity). Details for the functions of thevarious modules shown in FIG. 2 may be gleaned from the furtherdescription of the present system, method and apparatus which isdescribed in more detail with respect to FIGS. 3-13.

3. Communication Subnetwork System

A specific communication subnetwork which includes an apparatus andsystem performing the modem link functions described above, in whichboth a network clock and a symbol clock are synchronized across a link,is described in detail in related U.S. Ser. No. 09/430,379, incorporatedhereinabove by reference. U.S. Ser. No. 09/430,379 describes acommunication subnetwork or system having base stations which eachprovide wireless links for transferring data between a plurality of endusers (e.g. FIG. 1, reference 120, 124 and 128) and a network through abroadband wireless link such as Modem Link 240. This link uses a limitedmedia, namely the wireless communication spectrum, which must be sharedby the plurality of users. In order to use the limited spectrum (orbandwidth) efficiently, the timing between the ends of the link,corresponding to master 200 and slave 250, is preferably very tightlysynchronized.

FIG. 3 shows details of the arrangement of links to a plurality ofusers, as is also described in the referenced wireless communicationsystem. The Base Station 300 includes apparatus and control to performmaster clock functions for both a network clock and a symbol clock. TheBase Station 300, via its antenna structure 302, transmits to (andreceives from) a plurality of users each having a corresponding CustomerPremise Equipment (CPE) station at 320, 322, 324, 326, 328, 330, and soon (note that these may each have a plurality of such CPEs).

The transmissions to and from the antenna structure 302 are directionalin nature, so that channels are limited to particular transmissionsectors, for example sectors 340, 350, and 360. Within the antennastructure 302, but not shown, are at least one directional antenna foreach sector. There may be a plurality of directional antennas servingany one sector, and there may be one or more standby antennas for eachsector as well. Each directional antenna may be packaged together withelectronics which provide up-conversion, filtering and poweramplification of signals received by cable from the base station 302,the combination forming an “outdoor unit” (ODU). Of course, many othersatisfactory configurations can be designed. Transmissions within aparticular sector, such as sector 340, are limited to the CPEs 320 and326 which are located within the transmission scope of that sector.Similarly, transmissions in sectors 350 or 360 are limited to CPEs at324 and 326, or at 328 and 330, respectively. The transmissions betweendifferent sectors are independent of each other. Such “sectorized”transmission permits spectrum reuse within a narrow area, thus providingmore bandwidth to service particular users. This arrangement limits thenumber of users which are multiplexed onto a single wireless link andthus must share the capacity of that link.

Within sectors, the downlink transmissions are multiplexed, whilebidirectionality is managed through adaptive time division duplexing.Each CPE has a distinct “virtual” connection, or channel, (e.g. 342)within its sector (e.g. 340). Since FIG. 3 arbitrarily represents threesuch channels per sector, there may be one CPE at 320 and two at 322,one at 324 and two at 326, and so on. There is generally a one-to-onecorrespondence between virtual channels 342-346 and the CPEs at 320 and322 in sector 340, between virtual channels 352-356 and the CPEs at 324and 326 in sector 350, and between virtual channels 362-366 and the CPEsat 328 and 330 in sector 360. These are merely representative; eitherfewer or significantly more virtual channels are possible within anyparticular sector, particularly if multiple frequencies are available.(If a plurality of carrier frequencies is available, the availablefrequencies may be allocated as needed between CPEs within a sector.)

Within each sector, communications are bidirectional on the basis ofAdaptive Time Division Duplexing (ATDD). All CPEs within a particularsector receive the same transmission from the antenna structure 302 ofthe Base Station 300 during a downlink portion of a time frame, while asecond portion of the time frame is used for uplink communications fromCPEs to the Base Station 300. The frame duration is preferably constant,but the proportion of time within the frame which is allocated fordownlink versus uplink transmissions is varied according to the needs ofthe channels served. Uplink transmissions are preferably time divisionmultiplexed, and each separate CPE in a sector will be allotted a uniquetime slot if they need to uplink data.

FIG. 4 shows an exemplary equipment arrangement in the Base Station 300,which may be referred to the “indoor unit” (IDU). The IDU equipment mayprovide signals to the ODUs, for example by sending signals from modeminterface controller card (MICs) 411-416 on a cable to each ODU at anintermediate carrier frequency. Each of the sectors, e.g. 340, requiresat least one distinct ODU having an antenna and preferably a frequencyconverter. Each of the ODUs may be connected to a distinct modeminterface controller (MIC) 411-416 corresponding thereto (although morethan one MIC may be built on the same physical printed circuit board).Note that the six referenced channels are merely exemplary, and werechosen for consistency with the six sectors which are arbitrarily shownin FIG. 3; more or less sectors may be provided for any particular basestation 300.

Signals received by the ODUs within the antenna structure 302 arefrequency down-shifted and delivered to the corresponding MICs 411-416.Those skilled in the art will understand that the functional stepsrequired for raising the modulated data signal to the transmissionfrequency, as well as the functional steps required for receiving anddownconverting received signals, can be divided many ways betweendifferent components. The MICs corresponding to each ODU may besupplemented by one or more standby MICs, or sMICs 418, which may bearranged for connection to an ODU in the event of a failure of eitherthe MIC or ODU serving a particular sector.

The base station IDU also includes at least one backhaul interface 442for physically connecting to a communication line. It will typicallyalso include at least one Network Interface Controller (NIC) card 432,434, for controlling the connection to one or more incomingcommunication lines. Cards 432, 434 may each include a plurality of suchNICs, or (to the same effect) may include a NIC capable of controlling aplurality of network interfaces. For example, each NIC card may handlefour T3 or E3 connections which provide data in a fixed-length packethaving an ATM protocol. However, many other types of connections may beimplemented in NIC cards, including variable-length IP packetconnections, ethernet connections, and so on. There may be a pluralityof backhauls, and each backhaul may be a wire line, an optical line, amicrowave connection, a satellite link, or any other high capacity dataconnection to a data router (not shown) which in turn interfaces to theInternet and/or to other wide area networks, such as the publictelephone network.

Particularly in the IDU of the base station 300, it is useful to includea separate Control Interface Card CIC 422. The CIC may be connected to aremote terminal for control data entry, such as through a Control I/Ocard 428, which might permit connection to an Ethernet or otherhigh-speed local area network which in turn is connected to a terminal.Again, many other arrangements are possible; for example, a multitude ofdifferent local area networks (LANs) or wide area networks (WANs) may beused to connect to the controlling terminal, or it may be connected by adedicated line, or even integrated with the CIC or other electronics ofthe IDU. For a fully redundant system, a standby CIC sCIC 426 may beprovided. Cables from the IDU to the network, and other cables from theIDU to the ODUs in antenna structure 302, are not shown. It is alsopossible to include other functionality in the base station, such as adirect broadcast satellite receiver equipment, and a video server andcentral computer 430 to perform functions such as high level set-up andmaintenance of service provision to individual customers, systemcomponent failure detection and correction, and other high levelfunctions.

Thus, in the exemplary embodiment shown in FIGS. 3 and 4, a Base Station300 may maintain wireless links with a large number of CPEs each havingtheir own virtual channel. The physical communications are sectorized,with one or more CPEs within each sector which are multiplexed onto anadaptive time division duplexed link. The following discussion regardingsynchronization of clocks across the link will deal with only one suchvirtual channel between a base station (typically master side) and a CPE(typically slave side). Those skilled in the art will have no difficultyextending the synchronization to a multiplicity of CPEs linked to a basestation 300, as are described above.

4. Symbol Clock Synchronization Across a Link

The preferred communication subnetwork system preferably employs anAdaptive Time Division Duplex (ATDD) technique for communication acrossthe wireless link. ATDD is preferably implemented in a framed system inwhich communication bursts take place periodically, and the burst perioddefines the time boundaries of a frame. Downlink communications from thebase station to the CPEs take place during one portion of each frame,and since the technique is adaptive, that portion is variable in length.

The base station also preferably employs a variety of modulationtechniques, sometimes in combination with a variety of error correctiontechniques, to direct data to particular CPEs. The combination ofmodulation and error correction creates a particular robustness level.CPEs cannot reliably read transmissions which are not sent with at leasta particular level of robustness. Therefore, CPEs will effectivelyreceive transmissions only during a downlink portion of a frame, andonly during that part of the downlink portion when the robustness levelis adequate. Thus, clock synchronization should work even when the slaveend of the link receives transmissions only during only a small part ofthe frames.

In order to accurately identify symbols, the symbol clock of a CPE orslave will preferably be locked to within ½ symbol period to a BS ormaster symbol clock. It is progressively more desirable that the clocksbe phase locked to within ¼, ⅛, 1/16, or 1/32 of a symbol. Such levelsof phase locking will preferably be maintained even under adverse signalconditions. The system and method disclosed herein will maintain one ofthese levels of phase locking even if communication from the slave tomaster is completely absent from some frames; or if communication frommaster to slave is variable in length and occupies as little as0.000125, or even 0.00006, of the time in a given frame; or even ifcommunication from master to slave is interrupted for as much as fivetypical 1 ms frames. As an example, a symbol clock operating at 20 MHzmay be synchronized across a link through transmissions which occupyonly 25 symbol clock periods, or even 12 periods, sent once permillisecond.

4.a. Burst Modem and Preamble

Reference is made to FIG. 2 to describe further details of an exemplaryembodiment of apparatus for synchronizing a symbol (or other noncommon)clock across a communication link (or modem link). To provide lockinginformation despite variable periods of non-transmission, it ispreferred that the master side 200 of FIG. 2 of the modem link initiatecommunication bursts at fixed intervals expected by the slave. Themodulated signal 242 is provided by a master-side modem 214 in a burstwhich is initiated at preferably constant intervals which are defined bya precise number of periods of the master symbol clock CSym₁, 212. Inparticular, CSym₁, may operate at nominally 20 MHz, and the bursts maybe initiated at a periodic interval of 1 ms.

The beginning of a burst preferably includes a preamble which will berecognized by the slave. For simplicity, the preamble may be the samefor each burst. A 25 bit preamble is preferred, but is a tradeoffbetween bandwidth consumed by the preamble and the simplicity ofobtaining an accurate recognition of the preamble and thus a precisedetermination of the frame timing. Moreover, after phase lock isachieved, only 12 preamble bits are preferably used to maintain phaselocking.

4.b. Symbol Clock Synchronization System Blocks

FIG. 5 shows modules of both a master system and a slave system whichwork together to synchronize a symbol clock. On the master side 552, theexpected preamble module 502 is preferably configured to provide afixed, predetermined pattern of 25 bits. The preamble module 502 mayalso vary the pattern depending upon whether phase lock has already beenacquired, and upon other factors affecting the ability to acquire andmaintain a phase lock, such as weak signal strength or the presence ofelectrical noise. The preamble is preferably prepended to data in thebuffer module 504, which is configured to queue incoming data from thetransfer module 506 in preparation for transfer across the link.Transfer module 506 is configured to accept data from elsewhere in thesystem, such as from the ATM switch, and to structure the data intoframes, with data for particular CPEs placed in the frame atpredetermined locations known to the receiving CPEs. The master symbolclock module C_(Sym1) 510 may be any frequency which is convenient forthe system, for example a 20 MHz clock. The master symbol clock moduleC_(Sym1) 510 controls the rate that symbols are transmitted from thebuffer module 504 to the master modulator module 516. The Master modemmodule is configured to modulate the data onto a carrier frequency,which may be an intermediate frequency for convenience in transmissionto an antenna location. The data signal, thus modulated, will betransferred across the link from the Master 552 to the Slave 550. Thebuffer module 504 will output a burst of data after the counter module512 counts to a number of clock periods equal to a frame period,preferably 1 ms. The clock output from the master symbol clock moduleC_(Sym1), 510 to the counter module 512 is doubled to 40 MHz, and thecounter module 512 counts down from 40,000 before providing an enableoutput to the buffer module 504 to cause it to begin sending the datastream, including the preamble, to the master modem. The master modemmodule 516 modulates the data onto the carrier and causes it to betransmitted over the air to the slave side 550 of the link.

The slave modem module 552 is configured to demodulate and filter thetransmitted data. Its output goes to a burst correlator module 560. Theburst correlator module 560 is configured to compare the signal from theslave modem module 552 to a signal from the expected preamble module572. The expected preamble module is configured to provide arepresentation of the preamble expected from the master side. Thepreamble may be fixed, but is preferably selectable in coordination withthe master 552. A 12 bit preamble is employed, and during acquisition ispreferably sent twice separated by one bit during acquisition.

The burst correlator module 560 is configured to determine an arrivalinstant for the preamble signal from the master 552. This time point ispresumed to be a known number of master symbol clock periods after thearrival of the previous preamble, and is compared to an output from thecounter module 570. The counter module 570 is configured to output timeindications separated by a comparable number (preferably the samenumber) of slave symbol clock periods as the number of master clockperiods separating the sent preamble. Thus, the counter module 570presents an “expected time” for arrival of the preamble. The counting ispreferably reset upon arrival of a first preamble, and is thereafter notreset, so that all errors are cumulative.

The burst correlator module 560 is also configured to compare the timingindication from the counter module 570 to the arrival time determinedfor the preamble, and to output the difference as error output 574.Error output 574 in turn is input to the loop filter module 590, whichis configured to filters the signal and then applies it as a controlsignal to the slave symbol clock module 580. Slave symbol clock 580 isconfigured to respond to changes in the control signal by adjusting itsfrequency.

The burst correlator module 560 may be configured to determine thedifference between the expected and actual arrival time of the preambleby analog means, but preferably converts the signal from the slave modem552 into a digital representation. The burst correlator module ispreferably configured to sample the signal to provide a complex pair of10-bit samples at a multiple of the symbol clock rate. The multiple ispreferably 1, 2, or 4 for convenience, but need not be 2^(k), k aninteger, and need not even be an integer number. Other samplingapproaches may be used, and particularly other multiples of the clockrate. The tradeoffs, such as processing requirements versus the errorsignal accuracy and resolution, will become apparent to those skilled inthe art. Further details of the burst correlator module 560 arepresented below with respect to FIG. 6.

The output from the slave modem module 552 also goes to a detectionmodule 576 which is configured to determine the value of the bit streamcontained in the signal, and convey it as data out to the rest of theslave communication system. To do so, the detection module is configuredto further accept an input from the slave symbol clock 580 representingthe symbol clock rate, and a detection offset input 562 from the burstcorrelator module 560.

One skilled in the art will understand that the preamble merely needs tobe expected in cooperation between the master and slave, and need not beidentical each frame. The burst needs merely to be initiated at anexpected time, rather than being sent at fixed intervals as is done fordesign convenience in the preferred embodiment. As long as the slavesystem can recognize the preamble and knows when it should arrive, itcan generate an error indication to adjust its local symbol clockoscillator.

The functions of the modules can be performed in either hardware orsoftware. In the case that the signal transmitted across the link is ananalog signal, at least some hardware processing must be done until thesignal has been digitized. Thereafter, a designer will choose to usehardware or software on the basis of the particular application.

Those skilled in the art will appreciate that the functions of thedifferent modules may be arranged in an unlimited number of ways. Forexample, functions from different modules may be performed in the samephysical device. Indeed, all of the modules of the master side or of theslave side can be designed to be performed by a singleapplication-specific integrated circuit (ASIC). As another example,functions from any particular module need not be performed in a relatedphysical location with other functions of such module, but may bescattered into other modules, except that slave-side modules areseparated from master-side modules by the communication link. Finally,the functions of modules may be incorporated into a different number offunctional blocks, so that either more or fewer modules are apparentlyutilized in any actual embodiment without significantly changing thesystem.

4.c. Symbol Clock Synchronization—More Detailed Block Representation

It is preferred that the slave symbol clock be phase-locked to themaster symbol clock. It is helpful, toward this end, to enhance theresolution with which the burst preamble timing can be detected. Anytechnique can be used in conjunction with other aspects of thisinvention. For example, a classic technique involves supplying thereceived preamble to a bank of correlators, the other input of thecorrelator being given a time-shifted version of the expected preamble.The time shift applied to the expected preamble in the correlator foundto have the largest magnitude output is deduced to most accuratelyreflect the actual burst timing. The preferred correlator system moduleachieves a similar effect by different means.

4.c.1. Input Processing Blocks

FIG. 6 shows further details of the slave side of a symbol clocksynchronization circuit or system. The A/D converter 602 samples theincoming signal, which has been demodulated from the millimeter wavetransmission frequency to a 40 MHz center frequency, at 80 MSamples/swith 10 bits of resolution. The samples are clocked by the 4× frequencyoutput of clock multiplier 658. The signal is downconverted to DCbaseband at DQM 604, providing a sequence of complex pairs of 10-bit Ivalues and 10-bit Q values. The complex sequence is filtered in thematch filter MF 606. The match filter performs root raised cosinefiltering, providing full raised cosine filtering in conjunction withfiltering at the transmit side, and outputs 40 MS/s 10-bit complexvalues. The preamble correlation 610 is an output sequence of 10-bitreal numbers, at a 40 MSample/s rate, which reflect correlation of theincoming signal with an expected preamble signal. The 40 MSample/s rateis related as n times the slave clock rate, where n is preferably 2 as aconvenient compromise between processing burden and signal reproductionaccuracy. However, those skilled in the art will understand that n,while preferably integer, need not be 2^(m), m an integer, and indeedmay be other than an integer. This multiple sets the basic resolution ofthe correlation output to a correlation resolution, which as describedis 1/n time the slave clock period, that is, the correlation sampleperiod.

The preamble correlation output sequence 610 goes to interpolators,described below, and also into peak detector 622. Peak detector 622provides an edge only after the received signal matches the expectedpreamble; the edge output is delayed appropriately to indicate a timewhen the correlation output will be centered in the interpolatorregisters into which it will be shifted. The preamble correlation 610 isdescribed in more detail with respect to FIG. 7, and the peak detector622 is also described in more detail with respect to FIG. 9.

4.c.2. Preamble Interpolation Blocks

Since the sample rate is 40 MS/s, n times the symbol clock rate, thepreamble correlation 610 can only identify the incoming preamble timingto within 1/n or ½ of a symbol period (the symbol clock is 20 MHz).Accordingly, in order to lock the Symbol Clock VCO 650 more accuratelyto the master symbol clock reflected in the arrival time of thepreamble, more resolution of the precise preamble arrival time isdesirable. One output of the preamble correlation 610 is a correlationsequence of five 10-bit real numbers, centered around the highestmagnitude output peak from the preamble correlator.

The correlation sequence is shifted through a bank of five interpolators612-620. Each interpolator is a five-tap finite impulse response filter,and each essentially correlates the sequence with an impulse shifted intime by ⅛ symbol, or 1/160 MHz seconds, from interpolator tointerpolator. This ⅛ symbol is an interpolator resolution, which is asmaller time unit than the sample resolution. Each of theseinterpolators outputs a value, I0 from interpolation0 612, I1 frominterpolation1 614, I2 from interpolation2 616, I3 from interpolation3618, and I4 from interpolation4 620. The maximum select block 624identifies the largest magnitude interpolator output, which reflects theactual timing of the preamble signal to within the interpolatorresolution of ⅛ symbol period. That is, the timing of the correlationoutput is interpreted to be ¼ or ⅛ period earlier, right on time, or ⅛or ¼ period later than the nominal correlation pulse (which has aresolution of only the sample clock period, which is ½ symbol clockperiod).

4.c.3. Symbol Centering/Resolving Blocks

The preamble interpolation maximum select block 624 output is used toalign the symbol detection (not shown) to the center of the symbolsignals so as to best resolve each symbol. First, the delay selectionoutput 626 of the maximum select block 624 indicates the most accurate ½symbol delay to apply to synchronize the incoming symbol signals. Thisselected delay is then applied to all subsequent filtered 10-bit complexsample pairs arriving at the delay buffer 670 from the match filter 606(until the next preamble arrives).

Second, the interpolation selection output 628 from the maximum selectblock 624 indicates which interpolator filter should be used toeffectively synchronize the symbol signals to within ⅛ symbol period.This selection is then applied to all incoming symbol signals at symbolinterpolation bank 672. This is a bank of five interpolators, providingfive possible timing shifts from −¼ to +¼ symbol periods in ⅛ periodincrements. One of the five interpolators of bank 672 is chosen forapplication by selection output 628. This automatic adjustment ofinterpolation may be restricted to periods when the symbol clock is notphase-locked to within some phase range of the master. In a well-behavedsystem, phase lock to within ⅛ of a symbol should eventually beachieved. Thereafter, it has been found generally preferable to forcethe symbol interpolation bank 672 to use the same interpolatorconstantly, irrespective of selection output 628, so that the variousdelayed values in digital filters in the system remain valid, and arenot effectively shifted in time compared with more recent data.

After the incoming signal has been interpolated in symbol interpolation672, it enters circuitry to detect the value of each symbol. It enters adelay buffer 674, and a correlation gain and phase estimation 676, theoutputs of which are forwarded to an equalizer which reduces intersymbolinterference and derives a refined identification of each incomingsymbol. This filtering and symbol identification can be done by any ofthe means now known or hereafter developed for such symbol detection.

4.d. Slave Symbol Clock VCO Control Blocks

The voltage controlled oscillator symbol clock VCO 650 is the 20 MHzslave symbol clock which is to be synchronized to the master symbolclock. The output of symbol clock VCO 650 is doubled at frequencydoubler 648 to 40 MHz, and then clocks the VCO Counter 652. A furtherdoubling at frequency doubler 658 to 80 MHz establishes the input A/Dsample clock. To accomplish synchronization, of course, differences orerrors between the slave symbol clock 650 and the master symbol clockmust be detected. In order to phase lock, an error must be determinedwith a resolution able to reflect phase error.

4.d.1. First Order Error

VCO Counter 652 counts the number of slave clock periods which isexpected between preambles from the master; preferably, the period is 1ms and the VCO counter therefore counts modulo 40000. The VCO Counteroperates at a multiple q times the slave clock frequency; q ispreferably an integer, preferably 2^(k), k an integer, and preferably is2. However, q need not be an integer.

The counter is reset to zero when the first preamble arrives after along hiatus, and thereafter it is not reset. The counter input to theVCO control circuit therefore functions as an (effectively) infiniteintegrator, because all remaining error between the expected number ofcycles and the actual number of cycles is accumulated as a sum, and iscarried forward and applied to compensate the clock frequency.

Output 622 from the preamble correlation 610 is a preamble-indicatingedge having a fixed relationship to the highest output sample of thecorrelation circuit. As such, the preamble-indicating edge has a timeresolution equal to the sample period of the correlation circuit output.This edge is preferably delayed as needed, and is used (thus delayed) tolatch the outputs of the interpolation filters 612-620 when the highestoutput of the preamble correlation 610 has been shifted until it iscentered in the interpolation shift register. The samepreamble-indicating edge 622 is used to latch the value of the doubledVCO counter 652 into latch 654. The doubled VCO counter is initiallyreset upon receiving a first preamble-indicating edge 622, and operatesat the same modulo as the master symbol clock burst timing counter.Therefore, the latched value reflects the error between the master andslave clocks to within ½ symbol period. This error indication preferablyhas at least the same resolution of the synchronization signal receipttime as is imposed by the correlation sample rate. In this case, theresolution is 1/n slave symbol clock periods, where n is 2.

4.d.2 Second Order Error

This error indication, at half-symbol (or 1/n slave symbol clock period)resolution, is left-shifted two bits by left-shift block 642,effectively multiplying the error by 4 so that two LSBs can be appendedin adder 640. The LSBs are obtained at mapped block 638 by mapping anumber which reflects an addition of from −2 to +2, based upon theinterpolation selection 628 which indicates which of I0 to I4 is largest(and thus indicates a best interpolation). The mapped value provides aninterpolation of the position of the best correlation peak to within ⅛symbol period, significantly less than the ½ symbol period provided bythe correlation samples, and this mapped value is then added to theshifted value of the latch at adder 640.

4.d.3. Further Resolution

The output of adder 640 has a resolution of ⅛ symbol period. It may beapplied directly to a digital loop filter like 656 to drive the symbolclock VCO 650. However, further refinement is preferred. Therefore, theoutput of adder 640 is multiplied by 8, for example by left shifting 3bits in left shifter 636. Then, if I2 provides the best interpolation,i.e. has the largest interpolator output (indicating that the clocks areless than ⅛ symbol period mismatched from a time indicated by I2), thenthe differencer 632 will be enabled. Differencer 632 compares theinterpolator outputs adjacent to the best interpolation, in this casecomparing interpolation1 614 and interpolation3 616. Before comparing,the 14-bit outputs of the interpolations are left-shifted by 10 bits(i.e. multiplied by 1024). Then, the result of the difference betweenthese two shifted outputs is clipped so as not to fall outside the range−8 to +7. The three-bit number resulting from this clipped comparison isthen added to the (3-bit left-shifted) value from the ⅛ symbol errorvalue.

The output of adder 634 thus has a LSB resolution of 1/64 symbol period,which is substantially less than the interpolator output. This output isfed into the digital loop filter 656 to drive the symbol clock VCO 650.However, if I2 was not the largest interpolation output latched by thecorrelation edge, then adder 632 is effectively disabled, and its outputis held at zero. However, the error resulting from interpolation isstill left-shifted by three, even though no further estimation of theinterpolated synchronization signal receipt time is added.

4.e. Refined Correlation Block Diagram

FIG. 7 represents the functions performed by the correlation block 610of FIG. 6. The 10-bit complex number pairs provided at 40 MSamples/sfrom the match filter (606 of FIG. 6) enter at 702. It is important tounderstand that the single line at 702 represents both I and Q values.The samples are sequentially shifted through registers 703-708(representing 22 actual registers for each number). The twelvecoefficients for the 12-bit expected preamble are represented by 710(C12), 712 (C11), 714 (C10) and 716 (C1). These coefficients may berepresented as either 1 or −1, simplifying the multiplication by eachsample as it is shifted through the registers 703-708. The result ofthese multiplies is summed at sum block, and the output squared insquaring block to provide a single real value for the correlation.

However, in order to discriminate against noise, the noise energy isthen subtracted from the correlation real value. Each complex pairpresently being correlated—i.e. the current input at 702, and the valuein registers 704, 706 and . . . 708, is squared at squaring blocks 722,724, 726 and . . . 728, respectively. The resulting real values are thenadded in summing block 730. This sum of squared values reflects theuncorrelated energy of the sequence being tested for correlation. Theenergy is multiplied at multiplier 734 by −K 732, and the product isadded to the squared correlation value from squaring block 720. Thevalue of −K will depend upon scaling throughout the system, includingthe values of C1-C12. It is preferably selected such that the square ofthe correlation sum exceeds K times the energy sum only when a strongcorrelation is found, and thus the output 738 is positive only when apreamble correlation is detected. However, those skilled in the art willunderstand that other methods of distinguishing noise, and other methodsof detecting a correlation, may be used as well.

4.f. Digital Loop Filter Block Diagram

FIG. 8 shows the digital loop filter 656. Input 802 arrives from adder634 of FIG. 6, and has an LSB value of 1/64 symbol period. It is summedwith positive feedback output 810 at summing block 804, the result ofwhich is clipped to between −4194304 and 4194303 in clipping block 806to prevent rollover errors. After a one period delay 808, this value ismultiplied by K_(i) 806, which has a value of 1/2048 (with rounding).The resulting product is added, at summing block 808, to a product takenat multiplier 810 of input 802 by K_(p) 812. K_(p) is preferably 0.5.The sum developed at summing block 808 is applied to the digital toanalog converter (DAC) 814. The analog output 816 from the DAC 814 isthen connected to the slave symbol clock VCO (650 of FIG. 6). Thoseskilled in the art will appreciate that most items described in thisembodiment, and especially the particular gain numbers 806 and 812, willpreferably be varied in accordance with the LSB resolution value, theVCO gain, the sample rate, and other circuit circumstances andperformance needs.

4.g. Interpolators

The interpolators such as 612-620 of FIG. 6 shift incoming correlationsequence data through a shift register, multiply each register output bya coefficient, and sum the result. The interpolators 612-620 use fivecoefficients in order to detect a slightly offset correlation sequencepeak. The five coefficients currently used, for interpolation0 tointerpolation4, respectively, are: −29, 151, 151, −29, 6; −13, 64, 226,−31, 6; 0, 0, 255, 0, 0; 6, −31, 226, 64, −13; and 6, −29, 151, 151,−29. The results may be scaled as desired for convenience, for exampleby right-shifting eight bits.

4.h. Peak Detector

The peak detector 622 of FIG. 6 is shown in more refined block detail inFIG. 9. The input 902 takes the 10-bit real values output from thepreamble correlation 610 (FIG. 6) and shifts them through shiftregisters 904, 906 and 908. At each point the value is multiplied by thecorresponding coefficients 910, 912, 914 and 916 (C1 to C4,respectively). The preferred value for the coefficients is 0.75, 0.25,−0.25 and −0.75 respectively. The outputs are summed at 930, whichreflects a derivative between correlation output samples. The circuitthereafter deduces the location of a peak by identifying (over a fewsamples) a sample point which has a positive derivative before it, and anegative derivative after it. Sign determining block 932 outputs X(k),with 1 indicating that sum 930 is positive, −1 indicating that sum 930is negative, and 0 indicating that sum 930 is 0. Difference 936 is adifference between X(k−1), i.e. 932 delayed by single delay 934, and thepresent output X(k) of 932. That output is multiplied by 0.5 atmultiplier 938, (which may be practically implemented as a one-bit rightshift), resulting in D(k) 940. D(k) 940 can only be 1 if difference 936was 2, which requires that X(k)=−1 and X(k−1)=+1, and thus the leadingslope is positive and the trailing slope is negative, as required toidentify a peak.

D(k) 940 equal to “1” suggests a peak, but is further discriminated byestablishing that correlation output r(k−2) is greater than zero. Thisis accomplished by determining the sign of r(k−2) (the output ofregister 906) at sign block 948, which outputs 1, 0 or −1 as did block932. This result is added to one at sum 942, and then truncated one bitless at “multiplier” 944, which may be a right-shift operation. Thisfurther results in W(k) 946, which is equal to 1 only if r(k−2) ispositive. P(k) 960, the output from multiplier 950, is W(k)*D(k), andP(k)=1 identifies a peak. Delay lines 962 delay P(k) by the number ofsamples expected between redundant synchronization signals. These aresent particularly during acquisition, and when peaks are found separatedby precisely the expected period indicates more certainly that thesynchronization receipt time indicated by P(k) was the correct one. Thiswill be reflected when output 970 from “multiply” 964 P(k)*P(k−i) isequal to 1 (“i” is the number of sample clock periods expected betweenredundant synchronization signals).

5. Network Clock Synchronization Across the Link

As explained previously in respect of FIGS. 1 and 2, some datatraversing a link may arrive at a rate determined by a source networkclock, e.g. CNET_(A), 104. If the data must be delivered by User 124, itis useful for avoiding data loss to deliver the data at a ratedetermined by CNET_(D), 132, which is substantially the same asCNET_(A), 104. It is therefore desirable to determine a network sourceclock, such as CNET_(A), 104, at a node e.g. 116, and to communicate theclock to another node, e.g. 124, across a link, 122, which communicatessynchronously on the basis of a communication clock such as thesynchronized symbol clock described above. In FIG. 2, the network clockis available to the master side 200 of a communication link as CNet₁,216, and it is desirable to synchronize a corresponding CNet₂ 264 on theslave side 250 of the link, preferably without consuming much bandwidthin the process.

The network clock will generally not be available for explicitcommunication across the link, because such explicit communicationgenerally requires too much of the available communication media,whether optical, wired or RF wireless. Moreover, the network clock willin general be entirely asynchronous to the (e.g.) symbol clock. Thesymbol clock (or any such clock separate from the network clock) isindependently synchronized across the link, as described in the previoussections. It may be unnecessary to phase lock the network clock, sincein many instances a frequency lock will suffice. If a first independentpair of clocks is synchronized across the link, as described above, thenone may efficiently synchronize a second, independent pair of clocksacross the same link by conveying data to the slave side which reflectsa relationship on the master side between the first (synchronized) clockand the second independent clock. Thus, the synchronized symbol clocksdescribed above may provide part of a solution for synchronizingconstant bit-rate (CBR) data transfers across a communication link.Synchronized clocks, such as those described, may serve as “noncommonclocks” which may be leveraged to synchronize other independent clocksacross the same link.

A noncommon clock compare (NCC) algorithm synchronizes independentclocks (e.g. network clocks) on each side of a communication link byleveraging a previously established relationship between two otherindependent, noncommon clocks on each side of the link (e.g. symbolclocks). Thus, four independent clocks are involved—each side of thelink has both a noncommon clock and another clock. For example, anetwork clock (typically reflecting a rate at which a source of CBR datais being provided for communication across the link) may bereconstructed on the receiving side of a communication link by causingits relationship to a noncommon symbol clock local to its side of thelink to match a relationship between a network clock on the transmittingside and a symbol clock local to that transmitting side. The symbolclocks will first be synchronized across the link, for example asdescribed above. The (e.g.) symbol clocks are referred to as “noncommon”clocks because, although they are synchronized across the link asdescribed above, they are inherently independent of each other. Theircontinued synchronization relies on nearly constant communication acrossthe link.

A separate and independent (e.g. network) clock on a first side of thelink will be compared to the noncommon, e.g. symbol, clock local to thefirst side, and a first clock relationship determined. The second sideof the link will create a local analog of the network clock. Thesecond-side network clock analog will be adjusted to have substantiallythe same relationship to the second-side version of the noncommon clockas was determined to exist between the network clock and the noncommonclock on the first-side.

The ensuing discussion addresses a transmit (Tx) or master side of thelink and a receive (Rx) or slave side of the link with respect to thenetwork clock. The master side is typically the base station (BS) sidein an exemplary embodiment. Therefore, with regard to the network clockthe terms BS, Tx and master are sometimes interchanged. The slave side,in the exemplary embodiment, is typically on the customer premiseequipment (CPE) or receive side of the link. The master or transmit(e.g. BS) side is the side having access to a source network clock. Thesource network clock is consistent with the rate at which constant bitrate (CBR) data is being sent from its source. The slave or receive(e.g. CPE) side is receiving the CBR data, perhaps mixed with othernon-CBR data, and must cause the CBR data to be clocked out at thesource rate in order to prevent overflow or underflow of buffers.

The ensuing discussion is directed to clocks present at the ends of acommunication link, as distinct from clocks which may be distributedaround a generalized network. Accordingly, a different subscript is usedfor certain references, such as CNet, to avoid confusion with earliergeneral comments regarding C_(NetA), C_(Net1), etc. The network clockswill be designated C_(NetS) for the slave side, typically the receivingside of CBR data, and C_(NetM) for the master side, typically the sidetransmitting CBR data to the receive side. Note that the master side forthe network clock need not be the same as the master side for the symbolclock. In fact, in some circumstances both sides of a link may be anetwork clock master side for different data streams, i.e. for datatransmitted across the link from that side. Accordingly, those skilledin the art will appreciate that the following discussion may properly begeneralized to encompass data travel in either or both directions, for aplurality of different network clocks, and to master sides (forparticular data streams) which are on either side of the link, or evenon both sides of the link.

5.a. Master Noncommon Clock Error

The following discussion is directed to one side or node of acommunication link, as described above, which functions as a master forpurposes of synchronizing the network clock. General reference may bemade to FIG. 10. On such network clock master side, a noncommon clockerror period (NCP) is a period of time T_(NCP) defined by the durationof a selected number N_(mnc) of master network clock C_(NetM) cycles atthe network clock frequency f_(Net). Thus, T_(NCP)=N_(nmc)/f_(Net). Thenumber of noncommon clock cycles (at noncommon clock frequency f_(nc))during a NCP is referred to as the noncommon clock error (NCE). The NCEof the master (or transmit-side) network clock to its local noncommonclock is indicated as TxNCE, while the slave-side analog is RxNCE.

The NCE has an expected or nominal value, and a minimum and maximumvalue which depend on the nominal values and on tolerances of thenetwork and noncommon clocks. The nominal value of the NCE isT_(NCP)*f_(nc), which is the same as N_(mnc)*f_(nc)/f_(Net). Thetolerances of the two clocks may be added (presuming they areuncorrelated) to determine the tolerance of their ratio. For example, ifthe network clock frequency f_(Net) has a tolerance of 100 ppm, and thenoncommon clock frequency f_(nc) a tolerance of 75 ppm, then the ratioof f_(nc)/f_(Net) will be known to within 175 ppm. Since N_(mnc) isknown, the range of the NCE will be readily calculated.

It is preferred that T_(NCP) be an integer number of frame periods ofthe system. In an exemplary embodiment, the frame period is preferably 1ms, and T_(NCP) may be, for example, 10, 50 or 100 ms. In thisembodiment, f_(Net) is nominally 8.192 MHz, and f_(nc), is nominally 10MHz. As an example, if T_(NCP) is 10 ms, N_(mnc)=10 ms*8.192 MHz=81920.The nominal NCE is 81920*10/8.192=100000. The range (assuming the giventolerance of 175 ppm) will be 36 (NCE=100000+/−17; 2 more are added forquantization error of the clocks).

In view of the known nominal values and tolerance, only P bits arerequired to unambiguously represent the NCE. For instance, in order tounambiguously represent values between 99975 and 100025, 2^(P)≧36, soP=6. Therefore, a P-bit counter which is counting noncommon clock cyclesis latched every N_(mnc) master network clock cycles. The differencebetween successive latched values reflects NCE unambiguously. Using thesame analysis for T_(NCP)=50 ms as a second example, the NCE rangeincreases to 176 (500000+/−88), and accordingly P=8 in that case. Thoseskilled in the art will be able to analogously select an appropriate Pfor the circumstances of particular circuits.

5.b. Master Clock Relationship Determination

We refer now more specifically to FIG. 10 to describe a representativeembodiment of the master side, with respect to a network or secondaryclock, of a communication link across which the network clocks will besynchronized by reference to their respective noncommon clocks. A clock1002 available to the master side reflects a CBR data source clock, andhas a nominal frequency of 32.768 MHz. Clock 1002 is divided down individer 1004 to produce the network reference clock C_(NetM) 1006operating at a nominal f_(Net) of 8.192 MHz. C_(NetM) 1006 is furtherdivided by N_(mnc) at divider 1008 to provide NCP clock edges 1010 at aperiod of T_(NCP) (=N_(mnc)/f_(Net)). A race condition resolver 1012avoids ambiguity when the TxNCE counter 1026 is being latched into TxNCElatch 1016. The symbol clock C_(sym) 1020, operating at a nominal 20MHz, is divided by divider 1022 to provide the master version of thenoncommon clock, C_(nm) 1024 at f_(nc), of nominally 10 MHz. The TxNCEcounter 1026 has at least P bits, the number of bits necessary tounambiguously represent T_(NCP)*f_(nc), as explained above. The P (ormore) bit output “transmit NCE” (TxNCE) 1018 from this circuit reflectsthe master noncommon clock error without ambiguity, based on knowledgeof the expected (nominal) value of NCE. Any fractional value of TxNCEwill be carried forward to subsequent NCE counts by virtue of the factthat the TxNCE counter 1026 is not reset; this avoids a random walk typeof error on the value of the TxNCE.

5.c. Slave Clock Reconstruction

FIG. 11 illustrates a flow chart for reconstruction of a slave networkclock which is on the other side of a communication link from the masterside which is described above. In this exemplary embodiment, TxNCE fromthe master will be conveyed to the slave about every T_(NCP) at inputblock 1102. The incoming sequence of TxNCE values is buffered in buffer1104. Test block 1106 determines when at least Q (typically 5 to 10)successive valid values of TxNCE are available, whereafter thedifference dTxNCE(n) 1114 between the present value TxNCE(n) and thepreceding value TxNCE(n−1) is determined, modulo 2^(P), at comparison1110. Thus, assuming P=8, the values of TxNCE are compared by a modulo256 compare. Generally, (A−B) mod N=(A−B) if A≧B, else=A−B+N. Theoutput, dTxNCE(n) 1114, is held constant if the two values input intothe modulo 256 compare 1106 are not both valid and consecutive. TheTxNCE values may be sent redundantly but without other error correction,and if these received redundant copies are not identical, or an error isotherwise detected, then they may both be presumed invalid.

After Q (typically 5 to 10) valid TxNCEs have been received, the slaveor receive NCE (RxNCE) calculator 1108 is enabled and calculates theslave value of NCE, RxNCE, from the slave network clock C_(NetS) and theslave noncommon clock C_(ns) in the same way as TxNCE is calculated onthe master (or transmit) side from C_(NetM) and C_(nm). In the exemplaryembodiment, both calculations use the same expected or nominal value forT_(NCP), though the skilled person will recognize that many otherrelationships can work equivalently. RxNCE enters buffer/delay/test1112. When two values are available for RxNCE, the difference dRxNCE(n)1118 between the last two values RxNCE(n) and RxNCE(n−1) is taken modulo2^(P) at RxNCE comparison 1116. The slave will not have invalid datafrom its own clocks; however, comparison to the master values may bepostponed until the slave noncommon clock C_(ns) is synchronized to themaster noncommon clock C_(nm). The actual NCE discrepancy dNCE(n)between the master and slave, dNCE(n) 1122, is determined by taking thedifference dRxNCE(n)−dTxNCE(n) modulo 2^(P) at difference block 1120.

The skilled person will recognize that there are many ways to processthis determined discrepancy dNCE(n) to obtain a drive value foradjusting the slave or receive-side network clock voltage controlledoscillator, C_(NetS) VCO 1190 to produce C_(NetS) at f_(NetS)=f_(NetM).In an exemplary embodiment, K successive values of dNCE(n) are summed atsummer 1140 to provide SUMdNCE. At block 1142, if counter h is less thanK (typically, K=8) then the current value of SUMdNCE is delayed, and hincremented, at block 1144 before adding the next value of dNCE 1122.Once h=K so that SUMdNCE includes K values of dNCE, a step size STEP isselected at block 1146 based upon the magnitude of SUMdNCE. Each of thesteps of block 1146 should be performed in order. If |SUMdNCE| is lessthan a first threshold T₁, then the step size is set to Slow (typically,a value of 2). If so, the next two tests will fail and may be skipped;if not, then if |SUMdNCE| is greater than T₁, STEP is set to Medium(typically, a value of 10). If |SUMdNCE| is also greater than a secondthreshold T₂ (T₂>T₁), then STEP is changed from Medium to Fast(typically, a value of 50); if the second threshold T₂ is not exceededthen STEP will remain at Medium. The thresholds T₁ and T₂ may be set tovalues of 2 and 3, respectively. In this exemplary embodiment, the VCOwill change the output frequency by 0.005 ppm times the value ofNCEdrive; thus Slow, Medium and Fast STEPs correspond to 0.01, 0.05 and0.25 ppm per step; however, the number of ranges and the STEP values maybe varied for different embodiments, as will be understood by skilledpersons. At block 1148, when h=K the sum SUMdNCE and the counter h areboth reset for the next addition at adder 1140. STEP is then output fromblock 1146 into multiplier 1150 until the next sum of K successivevalues of dNCE is accumulated. Meanwhile, each present value of dNCE(n)is simplified to “sign or zero” at block 1152 by selecting OutS=−1 ifdNCE(n) is less than zero, OutS=0 if dNCE(n) is equal to zero, andOutS=1 if dNCE(n) is greater than zero. OutS is multiplied by STEP atmultiplier 1150. A running total of this product of OutS and STEP isformed as NCEdrive 1160 by adding the previous value of NCEdrive 1160,delayed by delay 1158, to the product at adder 1156. Finally, the valueof NCEdrive is filtered in Filter 1170 and then input to C_(NetS) VCO1190 to produce C_(NetS) at f_(NetS), where f_(NetS) is on average equalto f_(NetM). For a typical network clock which is defining a deliveryrate for CBR data, such exact match of the average frequency is adequateto prevent overflow and underflow errors. However, the skilled personwill recognize that for some purposes a closer lock between the masterand slave (e.g. network) clocks will be advantageous, and that suchcloser lock may be obtained using the same basic noncommon clock comparetechnique shown here, with adjustments made to the described processingalgorithm to improve the speed and accuracy of the C_(NetS) frequencyadjustment.

FIG. 12 shows an exemplary embodiment in which filter 1170 isimplemented beginning with a first-order, 16-bit sigma-delta digital toanalog converter (DAC). NCEdrive 1160 enters DAC register 1202, where itremains until updated after the next TxNCE is processed. The 16-bit wordfrom DAC register 1202 is applied to the DAC 1200 generally.Specifically, it is input to adder 1204, where it is accumulated withthe previous output from adder 1204 by way of delay 1206. Also, at adder1208, either −32767 (if the output of quantizer 1210 is 1) or +32767 (ifthe output of quantizer 1210 is 0) is added from selector 1212 to theprevious output of adder 1204. Quantizer 1210 outputs “1” if its input(the output of adder 1204) is greater than zero, and outputs “0”otherwise. In the exemplary embodiment, the sigma-delta converter isclocked at 20 MHz, resulting in a PWM output 1220 which is “1” for atime proportional to the value of NCEdrive 1160. In an exemplaryembodiment, the voltage value of a “1” output is 5V, while the voltagevalue of a “0” output is 0V.

The skilled person will recognize that many alternative algorithms willperform the same basic tasks shown in FIGS. 10, 11 and 12, including ofcomparing differences between the network and noncommon clocks on eachside of the link, and also that all or any part of the steps of thealgorithm may be implemented in either software or in hardware (e.g.using a field-programmable gate array FPGA). FIG. 13 shows a division offunctions between hardware and software in an exemplary embodiment.Slave, or receive-side noncommon clock 1370, operating at nominally 20MHz, is controlled as described elsewhere in this application to lock tothe master or transmit-side noncommon clock, and it's output is an inputto the RxNCE calculator 1108. The FPGA 1310 accepts the NCEdrive 1160value provided under software control from general purpose computer1320. The FPGA 1310 incorporates the DAC register 1202, the value ofwhich enters the DAC 1200. PWM output 1220 exits the FPGA 1310 to enteranalog lowpass filter 1350, which has a bandwidth of 10 Hz or somewhatless. (The analog filter 1350 is incorporated in the Filter block 1170of FIG. 11). The output of filter 1350 is applied to the control inputof C_(NetS) VCO 1190, which operates at a nominal 57.344 MHz. Alsoincorporated in C_(NetS) VCO 1190 as shown in FIG. 11 is divider 1362,which divides the C_(NetS) by 7 before it is input to the RxNCEcalculator 1108. This calculator performs, on the slave or receive side,the functions shown in FIG. 10 for the master or transmit side. Theoutput from calculator 1108 is placed in RxNCE Register 1320, andcommunicated to the general purpose computer 1320. Computer 1320 thenperforms steps corresponding to the functions shown in FIG. 11, with theexception of RxNCE Calculator 1108, Filter 1170 and C_(NetS) VCO 1190.

In an exemplary embodiment, each count of NCEdrive adjusts the frequencyof the C_(NetS) VCO 1190 by about 0.005 ppm. For an exemplaryembodiment, the VCO 1190 has a pull-in range of +/−100 ppm, a controlvoltage range of 0 to 5V, a frequency accuracy of +/−32 ppm and drift of+/−30 ppm. If the accuracy of the master network clock C_(NetM) is Xppm, then the VCO needs to be set to have a control range, under thelisted conditions, of about +/−(100+32+30+X) ppm; if C_(NetM) has anaccuracy of 1.6 ppm, then the range would be about +/−165 ppm. Theexemplary embodiment, using a 16-bit converter, therefore has aresolution at NCEdrive of about 330 ppm/65536=0.005 ppm. The resolutioncan, of course, be changed for different embodiments.

Those skilled in the art will appreciate that the circuits describedabove are merely exemplary. In particular, a great deal of latitude isavailable as to how the functions are implemented. Most functions may beperformed in either hardware or software according to ordinaryengineering design decisions. Moreover, most of the circuits andfunctions described may be scaled for different clock speeds, filtercoefficients, filter sizes, preamble sizes, and resolution needs.

The invention has been described in exemplary embodiments and aspectswhich are not limiting. Rather, the scope of the invention is defined bythe claims which follow.

1. A system in a transmitting communication station to transmitinformation sufficient to enable a receiving station to synchronize aplurality of clocks at the receiving station to a correspondingplurality of clocks at the transmitting station, the system comprising:a noncommon clock; a modulator module including circuitry to modulate acarrier to contain data for transmission to the receiving station, themodulator module being configured to modulate the carrier with apredetermined pattern in a periodic manner, the period being defined bya predetermined number of cycles of the noncommon clock; and a secondaryclock module including a secondary clock which is asynchronous to thenoncommon clock, the secondary clock module being configured to obtaincomparison data reflecting a comparison between the secondary clock andthe noncommon clock, the data being derived from a count of either (i) anumber of secondary clock cycles occurring within a predetermined numberof noncommon clock cycles, or (ii) a number of noncommon clock cyclesoccurring within a predetermined number of secondary clock cycles, andprovide the comparison data to the modulator module for transmission tothe receiving station.
 2. A system in a receiving communication stationto respond to clock information received from a transmitting station byadjusting a plurality of clocks at the receiving station to track acorresponding plurality of clocks at the transmitting station asrepresented by the received clock information, the system comprising: ademodulator module including circuitry to demodulate a signal receivedfrom the transmitting station, the demodulator module being configuredto determine a precise time of arrival of a predeterminedsynchronization signal pattern, and to derive data including secondaryclock relationship information; a noncommon clock module configured toadjust a noncommon clock in accordance with a difference between thetime of arrival of the synchronization signal and an expected time ofarrival based on a known number of cycles of the noncommon clock; and asecondary clock module configured to determine clock comparison datareflecting a count of either (i) a number of secondary clock cyclesoccurring within a predetermined number of noncommon clock cycles, or(ii) a number of noncommon clock cycles occurring within a predeterminednumber of secondary clock cycles, and adjust a secondary clock frequencybased on a difference between the comparison data and the receivedsecondary clock relationship information.